This invention relates to examination and imaging of biological tissue using visible or infra-red radiation.
Traditionally, potentially harmful ionizing radiation (for example, X-ray or .gamma.-ray) has been used to image biological tissue. This radiation propagates in the tissue on straight, ballistic tracks; i.e., scattering of the radiation is negligible. Thus, imaging is based on evaluation of the absorption levels of different tissue types. For example, in roentgenography the X-ray film contains darker and lighter spots. In more complicated systems, such as computerized tomography (CT), a cross-sectional picture of human organs is created by transmitting X-ray radiation through a section of the human body at different angles and by electronically detecting the variation in X-ray transmission. The detected intensity information is digitally stored in a computer which reconstructs the X-ray absorption of the tissue at a multiplicity of points located in one cross-sectional plane.
Near infra-red radiation (NIR) has been used to study non-invasively the oxygen metabolism in tissue (for example, the brain, finger, or ear lobe). Using visible, NIR and infra-red (IR) radiation for medical imaging could bring several advantages. In the NIR or IR range the contrast factor between a tumor and a tissue is much larger than in the X-ray range. In addition, the visible to IR radiation is preferred over the X-ray radiation since it is non-ionizing; thus, it potentially causes fewer side effects. However, with lower energy radiation, such as visible or infra-red radiation, the radiation is strongly scattered and absorbed in biological tissue, and the migration path cannot be approximated by a straight line, making inapplicable certain aspects of cross-sectional imaging techniques.
Several different approaches to NIR imaging have been suggested in the past. One approach undertaken by Oda et al. in "Non-Invasive Hemoglobin Oxygenation Monitor and Computerized Tomography of NIR Spectrometry," SPIE Vol. 1431, p. 284, 1991, utilizes NIR radiation in an analogous way to the use of X-ray radiation in an X-ray CT. In this device, the X-ray source is replaced by three laser diodes emitting light in the NIR range. The NIR-CT uses a set of photomultipliers to detect the light of the three laser diodes transmitted through the imaged tissue. The detected data are manipulated by a computer of the original X-ray CT scanner system in the same way as the detected X-ray data would be.
Different approaches were also suggested by S. R. Arriadge et al. in "Reconstruction Methods for Infra-red Absorption Imaging," SPIE Vol. 1431, p. 204, 1991; F. A. Grunbaum et al. in "Diffuse Tomography," SPIE Vol. 1431, p. 232, 1991; B. Chance et al., SPIE Vol. 1431 (1991), p. 84, p. 180, and p. 264; and others who recognized the scattering aspect of the non-ionizing radiation and its importance in imaging. None of those techniques have fully satisfied all needs in tissue examination.
In summary, there continues to be a need for an improved system which utilizes visible or IR radiation of wavelengths sensitive to endogenous or exogenous pigments to examine or image biological tissue.